The Universal Mobile Telecommunication System (UMTS), also referred to as the third generation (3G) system or the wideband code division multiplexing access (WCDMA) system, is designed to succeed GSM. UMTS Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system. In the UTRAN architecture, the user equipments (UE) 150 of a specific cell 110, are wirelessly connected to a NodeB (NB) 130, which in turn is connected to a Radio Network Controller (RNC) 100, as illustrated in FIG. 1.
The evolution of UTRAN and other radio interface standards is strongly focused on packet access technologies, to support packet data services such as VoIP, where the main principle is that small data units or packets carry the data over the communication medium and each packet comprises a header describing the data. To support the use of delay sensitive packet-data services, increased data rates and reduced Round Trip Times (RTT) is a requirement. RTT is defined as the time it takes for a packet to get from a first machine to a second machine and back again. In order to allow for reduced RTT and increased data rates in a UTRAN, the Transmission Time Interval (TTI) is reduced from 10, 20, 40 or 80 ms down to 2 ms. The TTI is defined as the duration of data transmission where coding and interleaving is performed.
Although a short TTI is generally beneficial for upper layer protocols and applications, there is a downside as well: The reliability of the transmitted data (and thus the coverage) decreases with a shortened TTI, as a shortened TTI means reduced energy per information bit. One solution to this problem is to increase the transmission power and thus increase the energy per information bit. The transmission of data using 2 ms TTI thus requires relatively higher transmission power, but in a transmission power limited situation the transmission will be more vulnerable to errors than the transmission of data by using 10, 20, 40 or 80 ms TTI. With 2 ms TTI it is thus difficult to ensure similar coverage as with a longer TTI. The coverage is especially limited in uplink (mobile-to-fixed direction), since a handheld UE cannot have as high transmitter power as the network side.
A widely-known solution to this coverage problem is to employ retransmission protocols, which means that receiving side requests for retransmissions from the transmitting side until the packet is successfully received (or the maximum number of retransmission is reached). A further improvement is to combine the retransmission protocol with soft-combining functionality where the receiver do not discard erroneously received packets but buffers their soft-bit values and combines these values with the soft-bits values of the retransmitted packets. This if often referred to as Hybrid Automatic Repeat Request (HARQ) with soft combining.
HARQ is a combination of forward error-correcting (FEC) coding and Automatic Repeat Request (ARQ). In FEC coding, redundancy is introduced in the transmitted signal. Parity bits are added to the information bits prior to the transmission, and the parity bits are computed from the information bits using a method given by the coding structure used. In an ARQ scheme, the receiver uses an error-detecting code to detect if the received packet is in error or not. If no error is detected, a positive acknowledgement (ACK) is sent to the transmitter, and if an error is detected, a negative acknowledgement (NAK) is sent. After a NAK, the transmitter will retransmit the same information again. HARQ thus uses FEC codes to correct a subset of all errors and relies on error detection with retransmission for handling the rest of the errors.
To reduce the delay introduced by the HARQ retransmissions, one solution is to allow a pre-defined number of retransmissions that are transmitted without awaiting the ACK or NAK between them. These so called autonomous retransmissions can be transmitted in consecutive TTIs, or in certain pre-configured TTIs that are not consecutively transmitted. If it turns out that the pre-defined number of autonomous retransmissions was not enough to get the data packet through, the UE will receive a NAK, and will then have to continue retransmitting (e.g. either ordinary HARQ retransmissions or another set of autonomous retransmissions) until it receives an ACK from the NodeB in response to a successful reception of the data packet (or until the maximum number of retransmissions is reached).
Although autonomous HARQ retransmissions can somewhat alleviate the above-described coverage problems, a fixed number of autonomous retransmissions will in some cases result in an excessive number of retransmissions when the UE is in a favorable situation in the cell. In general, an excessive number of HARQ retransmissions is a disadvantage, as the requirements on the receiver resources become strong at the network side, which translates into a high cost. If an amount of UEs are always performing a large number of HARQ retransmissions, the cost for the provided service becomes high. A large number of HARQ retransmissions also increase the delay, which is undesirable for real-time services such as voice e.g. On the other hand, the fixed number of autonomous retransmissions will sometimes not be enough to get a correctly received data packet, and some extra transmissions will therefore be needed. This increases the delay as well.